1. Field of the Invention
The present invention relates, in general, to a process for preparing metallic fibers and, more particularly, to the mechanical deformation and plastic deformation of metal powder into metallic fibers which are suitable for use as fillers for electrically conductive paints, pastes and plastics, and for use in metal catalysts and electrodes, both requiring large contact areas, sound-absorbing plates and filters.
2. Description of the Prior Art
Electrically conductive paints or plastics, which are extensively used for electromagnetic wave shielding at present, are made of mixtures of paints or resins and conductive fillers, which are typically exemplified by metallic powder, metallic flakes, metallic fibers, and metal-coated glass fibers. In view of the fact that the electrical conductivity of conducting plastics or paints is dependent on the connection between the fillers themselves in the binders, fiber-type fillers are increasingly coming into general use by virtue of their excellent connectivity.
With applications for catalysts and electrodes, metals are required to have large specific surface area in order to increase the reaction rates in which the are involved. When existing in a fiber phase, metal can have a maximum specific surface area.
Metallic fibers also have an application for filters for special conditions, especially high temperatures, under which synthetic fibers or natural pulp fibers are difficult to use.
To be useful as conductive fillers, metallic fibers are required to have as small a diameter as possible, preferably, a diameter of 50 xcexcm or less. When serving as a filler, a metallic fiber with a smaller diameter can be mixed at a lower fraction with a binder such as a resin or a paint. Such thin metallic fibers cannot be prepared by ordinary wire processing methods, such as wire drawing method. Thus far, various processes have been developed for preparing metal into thin fibers which are useful for such purposes.
Of them, a bundle drawing process, a vibrational cutting process and an in-rotating water melting spinning process are effective in preparing metallic fibers for conductive fillers.
By the bundle drawing process, metallic fibers with a diameter of as small as 10 xcexcm can be prepared. Another advantage of the bundle drawing process is that the metallic fibers can be freely controlled in length through later cutting steps. However, the bundle drawing process has the drawback of incurring large expense during the bundling of wires, the repetition of wire drawing, and the separation of wires after final drawing.
Advantageous as it is in that it is conducted simply and applicable for almost all materials, the vibrational cutting process suffers from the disadvantage of being unable to reduce the diameter of metallic fibers to below 50 xcexcm. Using only 5 wt % of the metallic fibers with a diameter of 10 xcexcm, which can be obtained by the bundle drawing process, plastics are able to be of sufficient electrical conductivity. On the other hand, at least 35 wt % of the metallic fibers prepared by the vibrational cutting process is required to make a plastic electrically conductive.
Over the above two processes, the in-rotating water melt spinning process has the advantage that it is less costly. One problem with the in-rotating water melt spinning process, however, is the limitation of the diameter of the prepared metallic fibers to 30 xcexcm or greater owing to the surface tension of molten metal streams jetted.
As mentioned above, it is difficult for such conventional metallic fibers-preparing techniques to avoid the problems associated with fiber diameters and production costs.
Therefore, it is an object of the present invention to overcome the above problems encountered in prior arts and to provide a process for preparing metallic fibers at low production cost and from most of the metals which are of plastic deformability, including silver, copper, aluminum and iron as well as precious metals such as palladium and platinum.
It is another object of the present invention to provide a method for preparing metallic fibers, in which metal particles are prevented from being bonded to each other during their plastic deformation, thereby easily separating individual metallic fibers from each other during the drawing process.
Based on the present invention, the above objects could be accomplished by a provision of a process for preparing metallic fibers, comprising the steps of: pre-treating metal powder of a predetermined size such that finally obtained metallic fibers can be separated with ease; elongating the pre-treated metal powder at a predetermined ratio; and separating metallic fibers from the elongated metallic material.
For optimal effectiveness, conductive fillers are required to have a length of 1,000-20,000 xcexcm and a diameter of 10-20 xcexcm for electrically conductive plastics, catalysts, and electrodes, and a length of 10-20 xcexcm and a diameter of around 5 xcexcm for electrically conductive paints.
Generally, when being prepared in a spraying process, metal powder has a diameter of 30-300 xcexcm. On the other hand, metal powder with a diameter of 1-10 xcexcm can be obtained by a chemical process. With such properties of the preparation processes of metal powders in mind, the present inventors take advantage of utilizing metal extrusion through which metal powder can be formed into wires at an extrusion ratio of several hundreds at the maximum. This means that, metallic fibers ranging in diameter from 1 to 50 xcexcm with a length of 10-500 xcexcm, which are suitable for use in electrically conducting plastics, catalysts, and electrodes, can be obtained by extending a suitable size of metal powder at an appropriate rate through a elongating process such as an extruding process.
The metallic fibers that the present invention can prepare are not limited to specific kinds. In other words, almost all metal materials are usable to prepare metallic fibers in accordance with the present invention. For instance, Pt powder, Pd power, Al and Al alloy power, Ag and Ag alloy powder, Ni and Ni alloy powder, Cu and Cu alloy powder, Ti and Ti alloy powder, Co and Co alloy powder, Fe and Fe alloy powder, Ni-, Ag-, Cu-, Au-, or Pt-coated metal powder, or mixtures thereof may be used as raw materials for the metallic fibers of the present invention. Also, metallic fibers can be prepared from stainless steel powder.
In accordance with the present invention, a pre-treating step is adopted to prevent metal particles from being bonded to each other during their plastic deformation, thereby easily separating from each other the metallic fibers obtained after a elongating step. In regard to the pre-treatment, there are three routes: (1) pre-oxidation of the surface of the metal particles; (2) coating of heterogeneous metal on the surface of the metal particles; and (3) mixing of the metal powder with salt, oxide or carbon. Detailed explanations will be given of each pre-treatment case, below.
When the pretreatment takes the pre-oxidation route, the surface-oxidized metal power is molded at room temperature by compression and extruded at an extrusion ratio which is determined in consideration of the required length and diameter. The extruded metal fibers are immersed in an acidic solution which does not etch the fiber phase of the metal powder, but selectively peels the oxide coating on the metal powder, so as to leach the oxide remaining between the metal fibers. As a result, metal fibers are separately settled down.
For the coating route, the same procedure as in the pre-oxidation route is repeated, except that the etching solution is so selected as to prefer the coating layer to the metal power.
After being extruded, the metal powder which previously underwent the mixing route is treated with a solution which can selectively dissolve the salt or the oxide, so as to isolate the metal fibers. When carbon black is used to mix with the metal powder, it is oxidized to CO and CO2, thereby readily separating metal fibers. Examples of available salts in the present invention include chlorides such as sodium chloride, barium chloride and potassium chloride; sulfates such as potassium sulfate and sodium sulfate; carbonates such as potassium carbonate; phosphates such as potassium phosphate; and fluorides such as sodium fluoride.
Over conventional techniques, the process of the present invention has the advantage of freely controlling the diameter and length of resulting metal fibers through the adjustment of the extrusion ratio and the selection of appropriate powder diameters and easily producing metal fibers through simple extrusion and separation; thus, surmounting the problems of conventional techniques, including shape limitation and high production cost.
The term xe2x80x9cmetal powderxe2x80x9d as used herein means collective metal or its alloy powder and it should be understood that no limitations are imposed on the production method of the metal powder. Although being described to be carried out through extrusion herein, the elongating process of the metal powder comprises a rolling process.